1. Field of the Invention
The present invention relates to an adsorption and removal method for efficiently removing nitrogen oxides (NOx) of low concentration contained in ventilation gas in various tunnels including various road tunnels, mountain tunnels, submarine tunnels, underground tunnels, and roads with shelter.
In various road tunnels, mountain tunnels, underground tunnels, roads with shelter and the like (these are collectively called "road tunnels" in the specification), particularly in those of long distance and heavy traffic volume, ventilation of a considerable volume is necessary for protection of health of passers and improvement of the distance of clear vision. Besides, in relatively short tunnels, in an urban or suburban district, the tunnel air must be aspirated and exhausted (ventilated) as means of preventing air pollution by carbon monoxide (CO) or NOx, etc. concentrated at the entrance and exit.
However, if the ventilation gas is directly scattered around, it is not regional environmental improvement, but is a mere spread of automotive exhaust and expands the highly polluted area in the urban or suburban district. As the pollution control means of existing roads, where tunnels and shelters are installed, the situation is exactly the same.
The invention relates to an adsorption and removal method for efficiently removing NOx of low concentration contained in the ventilation gas of such road tunnels.
As the characteristics of the ventilation gas of various tunnels, the concentration of NOx contained therein is low, about 10 ppm or less, the gas temperature is 100.degree. C. or less, or usually ordinary temperature, and the gas volume fluctuates significantly depending on the traffic volume.
2. Description of the Prior Art
The inventors proposed a purification apparatus of ventilation gas of road tunnel for the purpose of efficiently adsorbing and removing NOx of low concentration, which is a rotary NOx adsorption and removal apparatus mainly constituted with a honeycomb block NOx adsorbent rotor composed of plate adsorbent bearing Ru or Ce on TiO.sub.2, comprising a NOx adsorption zone, a preheating zone for preheating the unregenerated adsorbent ahead of the adsorption zone in the rotating direction, an adsorbent regeneration zone ahead of the preheating zone in the rotating direction, and a cooling zone for cooling the regenerated adsorbent ahead of the regeneration zone in the rotating direction (see the Japanese Laid-open Patent No. 3-258324). This apparatus is capable operating the entire system continuously, does not need frequent opening and closing of the valves and dampers, is easy to operate, and is excellent in durability.
However, the rotary NOx adsorption apparatus of this constitution has the following problems.
(1) A large mechanical strength is required on the sliding surface of the adsorbent rotor. (2) The sealing performance is poor on the sliding surface of the adsorbent rotor. (3) It is hard to manufacture rotors of large aperture. (4) The adsorbent rotor parts in the regeneration process do not contribute to the gas purification function, and a huge volume of adsorbent is needed for required performance. That is, about a half volume of the adsorbent rotor is always regenerated, and the adsorbent does not contribute effectively to the NOx adsorption. (5) The regeneration time and adsorption time are linked, and the times cannot be combined freely.
It is hence a primary object of the invention to present a NOx adsorption and removal apparatus solving all these problems.
In a purification apparatus of road tunnel ventilation gas, in most cases, the operation is limited to the daytime only when the traffic is heavy, and the operation is stopped or in stand-by in the midnight when the traffic volume is extremely small. Accordingly, the invention presents a system capable of extending the adsorption time significantly, by stopping continuous regeneration treatment of adsorbent by the rotary NOx adsorption apparatus and using the whole volume of adsorbent in adsorption.